Comments on Case Report: A fatal case of Aspergillus felis infection in an immunocompetent host

Received 10 January 2023; Accepted 07 March 2023; Published 03 April 2023 Author affiliations: Department of Veterinary Clinical Sciences and Centre for Animal Health and Welfare, Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong Kong, Hong Kong, SAR, PR China. *Correspondence: Vanessa R. Barrs, vanessa. barrs@ cityu. edu. hk

I have wondered why it is that apparently immunocompetent cats develop invasive FRS. Over-representation of Persian-related cat breeds raises the possibility of an uncharacterized genetic defect in fungal immunity or of impaired sinus drainage due to foreshortening of the skull (brachycephaly). In favour of the former, these same breeds of cats are also susceptible to other invasive fungal infections (IFIs), such as dermatophytic pseudomycetoma. There has been very limited veterinary research to investigate putative immunological defects. The findings from one study did not support a role for single-nucleotide polymorphisms (SNPs) in the coding regions of pattern recognition receptor genes, specifically Toll-like receptor (TLR) genes 1, 2 and 4, in the pathogenesis of fungal rhinosinusitis caused by Aspergillus species in cats [8] .
Regardless, invasive FRS also occurs in immunocompetent crossbred cats with no known underlying risk factors. An analogous condition, that of chronic granulomatous invasive fungal sinusitis, caused by Aspergillus flavus, has been reported for decades in immunocompetent or mildly immunosuppressed people from Sudan, India, Pakistan and Saudia Arabia [9]. The environmental burdens of A. flavus conidia are high in these regions [10]. Exposure to high environmental spore burdens may be the most logical explanation for the development of invasive FRS in some immunocompetent cats.
A role for species-specific fungal virulence factors in the development of invasive FRS in immunocompetent individuals must also be considered. In experimental models the A. felis type strain (isolated from a cat with sino-orbital aspergillosis) showed significantly higher virulence in Galleria mellonella larvae than Aspergillus fumigatus. In immunosuppressed BALB/c mice, A. felis showed slightly higher virulence than A. fumigatus. However, in a chronic granulomatous disease murine model, inoculation with A. fumigatus, unlike A. felis, was uniformly fatal, demonstrating decreased virulence in hosts deficient in production of reactive oxygen species [11]. Another study investigated changes on computed tomography of the head in immunocompetent cats with invasive and non-invasive FRS caused by any Aspergillus species, where the infecting isolate had been identified by PCR and sequencing of the internal transcribed spacer (ITS) regions (ITS1-5.8S-ITS2), partial β-tubulin and/or partial calmodulin genes [12]. There was a strong association between the infecting species and anatomical site of aspergillosis, with infections caused by A. fumigatus confined to the sino-nasal cavity, while cryptic species infections (mostly caused by A. felis OPEN ACCESS and A. udagawae) were associated with invasive sino-orbital disease, including orbital and paranasal soft tissue involvement and lysis of the orbital bone.
A further consideration is that A. viridinutans complex members are heterothallic but, unlike A. fumigatus, readily produce teleomorphs [5]. Whether there is any difference in the ability of asexual (conidia) or larger sexual spores (ascospores) to induce invasive infections, and the comparative localization of these inhaled bioaerosols, has not been investigated.
The antifungal susceptibility profile of the A. felis isolate reported by Parkes-Smith et al. is typical of most A. viridinutans complex members, i.e. low MICs/MECs of posaconazole and echinocandins, respectively, elevated MICs of voriconazole and itraconazole and variable MICs of amphotericin B. The combination of posaconazole and liposomal amphotericin B used in the report was also used to cure a cat with FRS caused by A. felis [1]. More recently, two cats with sino-orbital aspergillosis caused by A. udagawae and A. felis were cured with combined therapy including posaconazole, caspofungin and terbinafine and remained asymptomatic more than 2 years after presentation [13]. A human patient with X-linked chronic granulomatous disease who developed severe vertebral osteomyelitis caused by A. udagawae was treated successfully using posaconazole and caspofungin [14].
Finally, a shout-out to the One Health approach of Australian microbiologists and to the International Society of Human and Animal Mycoses (ISHAM), where sharing of comparative human and animal data has long been actively encouraged. Together, we can learn more.

Funding Information
The author received no specific grant from any funding agency.

Conflicts of interest
The author declares that there are no conflicts of interest.
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Major Comments: The author highlighted an interesting point that immunocompetent cats (particularly those of Persian-related breeds) may be predisposed to invasive fungal rhinosinusitis due to uncharacterized genetic defects or skull changes. Has there been any specific research related to this in veterinary medicine? It would be helpful to reference those studies. There have been limited studies in humans that have looked at the correlation between sinus cavity sizes and the occurrence of fungal balls (Michel et al DOI: 10.1111/coa.12813).
Response:Thank you for raising this point. There has been only limited research in veterinary medicine. One investigation looked at changes on computed tomography of the head in immunocompetent cats with invasive and non-invasive fungal rhinosinusitis caused by any Aspergillusspecies, where the infecting isolate had been identified by PCR and sequencing of the internal transcribed spacer (ITS) regions (ITS1-5.8S-ITS2), partial β-tubulin and/or partial calmodulin genes (1). There was a strong association between the infecting species and anatomic site of aspergillosis with infections caused by A. fumigatusconfined to the sino-nasal cavity, while cryptic species infections (mostly caused by A. felisand A. udagawae) were associated with invasive sino-orbital disease including orbital and paranasal soft-tissue involvement and lysis of the orbital bone.
Findings from another study did not support a role for single nucleotide polymorphisms (SNPs) in the coding regions of pattern recognition receptor genes, specifically Toll-like receptor (TLR) genes 1, 2 and 4 in the pathogenesis of fungal rhinosinusitis caused by Aspergillusspecies in cats (2). This information and references have been added to the manuscript.

Minor Comments:
The author reports that Aspergillus viridinutanswas isolated from rabbit feces in Frankston in 1954. It would be helpful to reference the study that this was reported in.
Response:Thank you for the suggestion -the reference has been added.
Please rate the quality of the presentation and structure of the manuscript Reviewer 2 Comments to Author: This short letter is a comment to a previously published case report (Case report: a fatal case of Aspergillus felis infection in an immunocompetent host, Access Microbiology, Volume 4, Issue 11), describing an unfortunate 18-year old succumbed from A. felis infection.
The manuscript was well written and statements were backed by citations, bringing up a few important key points about A. felis and its pathogenesis in humans and cats. A. felis was first described in 2013 and there are few published results on this pathogen. The author proposed that this pathogen needs more attention from microbiology and clinical fellows to share more data, to unveil pathogenesis mechanisms, and to demonstrate whether certain breeds of cats are more susceptible to A. felis infecitons.
Echoing the shout-out of the author of this letter, epidemiological investigation is recommended to be performed to identify where this patient got infected since he only has a healthy dog, whether there are any other individuals also got infected with possibly milder symptoms. Since this pathogen was recently identified and our understanding is poor, a better animal model mimicking infections would help obtain more knowledge on this pathogen and its related members.
Response:Thank you for the constructive comments.